1. Technical Field of the Invention
This invention concerns radiation-curable compositions, for example radiation-curable screen printing inks.
2. Description of the Prior Art
Hitherto proposed screen printing inks frequently include N-vinyl-2-pyrrolidone, especially when the inks are to be printed on to polymeric substrates. Indeed, N-vinyl-2-pyrrolidone has for a number of years been a key component of many ultraviolet light curable inks due to its inherent properties, for example acting as a diluent for viscous oligomers in such inks, and as a reactive monomer which cures with the ink. In practice, N-vinyl-2-pyrrolidone has been found to give a better reduction in ink viscosity than many acrylate monomers, for example phenoxyethyl acrylate, and tri-propylene glycol diacrylate.
In addition to acting as a curable monomer and a viscosity reducing agent, the polarity of N-vinyl-2-pyrrolidone has been used to enhance the wetting characteristics of inks when they are to be applied to plastics substrates, and its solvent power has contributed to the adhesion of these inks to such substrates.
These various properties of N-vinyl-2-pyrrolidone have led to its being used widely in screen printing inks.
Screen printable, radiation curable inks frequently include components with acrylate functionality because the double bond of the acrylate group is highly reactive in free radically initiated polymerization reactions. Although the vinyl functionality of N-vinyl-2-pyrrolidone is also reactive in free radically initiated polymerization reactions, it is generally slower to react than are acrylate groups.
Typically, radiation curable inks contain from 5 to 35 percent by weight of N-vinyl-2-pyrrolidone, such amounts usually giving the inks commercially acceptable adhesion, viscosity, wetting and curing/drying times. However, the presence of pigments in the inks can inhibit penetration of the curing radiation into the inks so that curing is not complete, leading to uncured components including acrylate and vinyl groups remaining in the ostensibly cured inks. Furthermore, uncured N-vinyl-2-pyrrolidone almost inevitably remains in view of the relative reactivities of acrylate and vinyl groups in the polymerization reaction.
Residual N-vinyl-2-pyrrolidone represents a major disadvantage for inks containing it, especially when, as often happens, printed polymer sheets are stored in a stack after printing and curing, or a roll of printed polymer is wound up and stored following printing and curing. The "dry" ink is then held in storage against the reverse side of another sheet or against the reverse of the web in the roll, and this can be under considerable pressure. This frequently leads to blocking of the sheets to each other or within the roll, and perhaps to the possibly less severe effect of marking or etching of the non-printed side of the polymer sheet or web where contact with the ink has occurred. These effects are the result of the relatively high volatility of N-vinyl-2-pyrrolidone and its solvent power. However, even slight marking of an expensive plastics sheet can be disadvantageous.
In some instances, these problems can be reduced by the use of the finest and/or semi-calendered screen meshes which give thinner ink deposits which cure more completely than ink deposits printed using coarser meshes. However, the problem often persists even when such meshes are used. An alternative technique would be to increase the duration of the cure, but this is not always commercially acceptable. Furthermore, if a print feels dry and has the desired adhesion to the printed substrate, the necessity for more fully curing the print is not obvious. For example, an opaque black ink printed on a plastics sheet and cured using low powered radiation might feel dry, and have acquired the necessary adhesion, but unreacted N-vinyl-2-pyrrolidone could in some circumstances damage a plastics sheet in contact with the ink.
Although the problem of residual unreacted N-vinyl-2-pyrrolidone is particularly acute with radiation-curable inks due to the screening effect of the pigment in the inks, the lower reactivity of N-vinyl-2-pyrrolidone compared with that of other free radically polymerizable components in radiation-curable compositions also leads to such residues.